Attenuator circuit



2 Sheets-Sheet. 2

INVENTOR DANIEL E HARNETT,

ATTORNEY ATTENUATOR CIRCUIT Filed April 7, 1938 D. E. HARNETT ET AL.

June 11, 1940.

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Patented June 11, 1940 ATTENUATOR CIRCUIT Daniel E. Harnett, Tuckahoe,and John F.

Farrington,

Flushing,

N. Y., assignors to Hazeltine Corporation, a corporation of DelawareApplication April 7, 1038, Serial No. 200,010

11 Claims.

This invention relates to modulated-carrier signal receivers and, moreparticularly, to an arrangement for reducing the maximum amplitude andthe range of amplitude variations of the input to a repeated stage ofsuch a receiver.

Under normal operating conditions, a modulated-carrier signal receiveris subject to the reception of signals of amplitudes varying within verywide limits. As a result, the input to the first repeater stage of thereceiver normally has a correspondingly Wide range of variation. Forweak signals it is desirable to maintain the transmissioncharacteristics of the response of the preselector between the antennaand the first re- 15 peater stage of the receiver at a maximum, in orderto raise the signals substantially above the noise level, thus toprocure the maximum use .ful sensitivity of the receiver. However, witha high gain in thepreselector, asthe strength of desired receivedsignals increases to such an extent that the swing of the grid of thefirst repeater stage includes a substantial nonlinear portion of thecharacteristic curve of the repeater, distortion of the desiredsignal-modulation envelope and cross modulation of the desired signalcarrier by strong undesired signals may result. This is particularlytrue in the case of receivers provided with automatic amplificationcontrol by which strong signals cause the grid to be biased toward thatportion of its characteristic which is most nonlinear. Undesired signalsas well as desired signals which reach the input electrodes of the firstrepeater stage have the effect of increasing the grid swing to such an.3 extent that these disturbingefiects result. It is well known thatsuch envelope distortion and cross, modulation are effects which cannotbe filtered out by succeeding selective circuits.

In general, the preselector, that is, the selector circuit between theantenna and the input circuit of. the first vacuum-tube repeater in thesignaltranslating channel, should be selective to pass a band of desiredmodulation frequencies sufiicient- 1y Wide to provide the desiredfidelity of repro .;,-,,duction. Generally speaking, if an attempt ismade to decrease the total desired and undesired signal input of thefirst repeater stage by discriminating against the undesired signalspassed by the selector, as by adjusting its band width, the to fidelityof reproduction is impaired to an undesirable degree. Furthermore, thisdoes not remove the envelope distortion of abnormally strong desiredsignals.

Variouscxpedients have heretofore been pro- 5 posed for automaticallyand adjustably attenuating the input to the first repeater stage of amodulated-carrier receiver in order to eliminate the disturbing effectsdescribed above. In cer' tain ofthese arrangement, adjustable impedanceelements, such as vacuum. tubes, have been con- 5 nected in circuit withthe preselector circuit of the receiver adjustably to damp suchcircuits, thereby adjustably to attenuate the input to the firstrepeater. This type of element, however, is usually inherently of highimpedance, which re- 10 stricts its application to a parallel connectionwith other elements of the preselector circuit. As thus connected, suchan adjustable impedance gener- ,ELIIY has an undesirable effect on theselectivity preselector circuits which aretunable over a wide range offrequencies, both. because it tends to restrict the tuning range andbecause its detuning effect may vary with adjustments of its attenuationeffect.

.It is an object of the invention, therefore, to provide an improved,simple, and economical attenuator which may be easily adjusted tocontrol the sensitivity of a modulated-carrier signal receiver in suchmanner as to eliminate the undesired effects described above.

It is a further object of the invention to provide an attenuator foradjustably attenuating the input to the first repeater stage of amodulated- 4n carrier signal receiver with freedom from undesired crossmodulation of the desired signal carrier by strong undesired signals onnearby frequencies.

It is a further object of the invention to provide a preattenuator ofthe type described which secures the desired result without affectingthe selectivity of a radio-frequency selector with which it isassociated.

In accordance with the invention, there is 5 provided in amodulated-carrier signal receiver including an input circuit and avacuum tube having an input circuit in the signal-translating channeloffthe receiver and comprising the firstsignal-tra-nslating tube of thereceiver, an attenuator comprising a first feed-forward coupling circuitfor coupling the input circuit of the receiver to the input circuit ofthe vacuum tube. The attenuator also includes a second feed-forwardcoupling circuit, substantially nonfrequency-selective over theoperating range, and including a repeater and terminal circuitsindividually coupled to the input circuit of the receiver and to theinput circuit of the vacuum tube, the second coupling circuit and itsterminal circuits providing a system substantiallynonfrequency-selective over the sideband range of the received signals.The second coupling circuit and its coupling to the input circuit of thetube comprises means for developing therein potentials opposite in phaseto those developed therein by the first coupling circuit. Means areprovided for controlling the repeating ratio of the second couplingcircuit in accordance with the amplitude of the received signals.

The novel features which are believed to be characteristic of thisinvention are set forth with particularity in the appended claims. Theinvention itself, however, both as to its organization and method ofoperation, together with further objects thereof, will best beunderstood by reference to the specification taken in connection withthe accompanying drawings in which Figs. 1. and 2 are circuit diagrams,partially schematic, of different forms of the attenuation control asapplied to a radio broadcast receiver of the superheterodyne type.

Referring to Fig. 1 of the drawings, there is shown a circuit diagram,partially schematic, of a complete modulated-carrier signal receiver ofthe superheterodyne type employing the inventionv This receivercomprises, in cascade, an antenna ground circuit I ll, a preattenuatingcircult II, a radio-frequency amplifier I2, a frequency changer oroscillator-modulator I31, an intermediate-frequency amplifier I4 of oneor more stages, a detector and A. V. C. supply I5, an audio-frequencyamplifier I6 of one or more stages, and a sound reproducer I1. Automaticamplification control is secured in a well-known manner by applying aunidirectional voltage derived from the A. V. C. supply I5 overconductor I5 to the control electrodes of oscillator-modulator I3 and ofone or more of the tubes included in the intermediate-frequencyamplifier I4, as shown in the drawings. Suitable operating potentialsare supplied to the tubes of the receiver from sources indicated as +56and +B.

Neglecting for the present the operation of the preattenuating means ofthe invention, the apparatus just described constitutes, in general, aconventional superheterodyne radio receiver, the operation of which iswell understood in the art; In brief, signals intercepted by antennacircuit I ll are translated to the input circuit of radio-frequencyamplifier I2 through the preattenuating circuit II, are amplified inradiofrequency amplifier I2, and converted to an intermediate-frequencysignal in oscillator-modulator I3. The intermediate-frequency signalsare amplified in intermediate-frequency amplifier I4 and are deliveredto detector I5 wherein the audio-frequency signals and the A. V. C.

biasing potentials are derived. These signals are,

in turn, amplified by audio-frequency amplifier I5 and supplied toloud-speaker I1 for reproduction. The automatic amplification controlbias is effective to maintain the signal input to detector I5 withinrelatively narrow limits for a Wide range of received signalintensities.

Referring now more particularly to the preattenuating system I! forcontrolling the input to radio-frequency amplifier I2, the arrangementcomprises, in general, a normal feed-forward coupling circuit betweenantenna circuit Ill and the input electrodes of radio-frequencyamplifier I2, and an auxiliary feed-forward coupling circuit in paralleltherewith and phased oppositely to the normal coupling circuit so thatvoltages of opposite potential are applied to the input circuit ofamplifier I2 by the two parallel couplings. In order to equalize thegain between the antenna and the, amplifier I2 over the tuning range,the antenna circuit is tuned near the lowfrequency end of the tuningrange by inductance 2I and the antenna capacitance, and resistors I8 andI9 are added to damp the resonance characteristic of this circuit. Thenormal coupling circuit between the antenna circuit III and the inputelectrodes of amplifier I2 is completed through a tuned circuitcomprising an inductance 22 and a tuning condenser 23, inductance 22being inductively coupled to inductance 2|. The auxiliary couplingcircuit between the antenna circuit Ill and amplifier I2 issubstantially nonfrequency-selective over the sideband range of receivedsignals and comprises a control repeater tube 24 having input electrodescoupled across resistor I9 through condenser 25 of low reactance, and anoutput circuit comprising an inductance 26 inductively coupled toinductance 22 with a polarity such that the coupling of the auxiliarycircuit is opposite in phase to that of the normal coupling circuit. Theoutput circuit of i'adimfrequency amplifier I2 is coupled to the inputcircuit of oscillator-modulator I3 through a transformer comprisingwindings 21, 28.

In order to provide a bias for control tube 24 which varies inaccordance with received signal intensities, thereby to control therepeating ratio of the auxiliary coupling circuit, there is provided anamplifying and rectifying channel including tubes 2!! and 3D. Amplifiertube 29 is coupled to the radio-frequency channel of the sistors 20 and39 being provided to aid in obtaining such characteristics. The diode35, 36 is provided with a load circuit comprising resistors 31, 38, 39across which there is developed a unidirectional voltage which varies inmagnitude in accordance with the signal input to amplifier 29.Theportion of this voltage developed across resistor 38 is appliednegatively to the grid of amplifier I2 through a filter, comprisingseries resistors l0 and shunt condenser II, to vary the amplificationthereof inversely in accordance 1 with the amplitude of receivedsignals, in a conventional manner. A second portion of the control-biasvoltage, developed across resistor 31, is applied positivelythrough afilter comprising series resistors 42 and shunt condensers 42' to thecontrol grid of control tube 24. In the absence of a biasing potentialderived from resistor 31, control tube 24 is biased by means of battery4-3 to a very low value of transconductance or beyond cutoff. There ispreferably provided a limiting circuit for tube 24 comprising the diodeelectrodes 44, 45 of tube 38 biased by a cathodebias battery 45 andconnected in shunt to resistor 31, across which the control bias fortube 24 is developed, through one of resistors 42,

In considering the operation of the system just described, it will beassumed that, in the absence of received signals or for signals of avery low amplitude, the tube 24 is biased to a low value oftransconductance or beyond i cutofi so that the auxiliary couplingcircuit of the invention is practically inoperative, the entire couplingbetween the antenna circuit l0 and the input electrodes ofradio-frequency amplifier l2 being that of the normal coupling circuitcomprising doubletuned transformer 2|, 22. However, as the amplitude ofthe received'signals increases, there is an appreciable input toamplifier 29, derived from the signal-translating channel of thereceiver through the transformer 3|, 32. The output voltage of amplifier29 is rectified by diode 35, 36 to develop bias potentials acrossresistors 37 and 38 which vary. in accordance with the amplitude ofreceived signals. The negative-bias voltage across resistor 38 isapplied through resistors 48 to the control grid of amplifier l2 todecrease its amplification as the intensity of received signalsincreases. At the same time, the voltage developed across resistor 31and applied positively to the control grid of control tube 24 throughthe circuit comprising resistors 42 serves to increase thetransconductance of the control tube 24. Inasmuch as the coupling due tothe normal coupling circuit and that due to the auxiliary couplingcircuit are opposite in phase, an increase in the transconductance oftube 24 serves to decrease the resultant coupling between the antennacircuit In and the input circuit of vacuum tube l2; that is, toattenuate the signal input to the first repeater of the receiver. Whenthe voltage developed across resistor 31 reaches a predeterminedamplitude, the limiter diode 44, 45 becomes conductive, effectivelygrounding the grid of control tube 24 through battery 45' andmaintaining its transconductance constant with further increases inreceived signal amplitude. This limiting circuit thus prevents thecoupling of the auxiliary circuit from over-balancing that of the normalcoupling. between antenna circuit l0 and the input of radio-frequencyamplifier l2 forreceived signal amplitudes of unusually large values andmaintaining the resultant coupling above some predetermined lower limit.It will be understood that the selector circuits in the mainsignal-translating channel of the receiver following the winding 3| aremore selective than those preceding the'winding 3| so that the majorportion of the selectivity of the receiver is provided by the circuitsin the signal channel succeeding the point to which the attenuationcontrol circuit is coupled; furthermore, the auxiliary coupling circuitcomprising the control repeater tube 24 and its terminal circuitscoupled to the antenna circuit 10 and the input circuit of the firstsignal-translating tube i2 is substantially nonfrequency-selective overthe sideband range of desired received signals and preferably, also,with respect to adjacent undesired signals. Hence, undesired signals aswell as desired signals are included in the input to amplifier 28 and tocontrol repeater tube 24.

The antenna attenuator arrangement of Fig. 2 is similar in principle ofoperation to that of Fig. 1, corresponding elements being identified bysimilar reference numerals. The arrangement of Fig. 2 includes anantenna-ground circuit comprising an inductance 46 inductively coupledto an inductance 4'! of a signal selector circuit which includes inseries tuning condenser 49 and resistor 48. A second signal selectorcircuit, comprising a series-connected inductance 41, tuning condenser5|, and resistor 50, is coupled to the input electrodes of aradio-frequency amplifier 52 having output electrodes coupled tomodulator l3. In this embodiment of the invention, the normalsignal-translating channel of the receiver comprises selector circuits41, 48, 43 and 41', 50, 5| coupled through resistor 52 and extendedportions of windings 41 and 41', effectively constituting tertiarywindings. The auxiliary control channel, having a coupling effectopposite to that of the normal coupling circuit, comprises a vacuum tube53 having an input electrode coupled to selector 41, 48, 49 through acondenser 54 of low impedance at signal frequencies and an outputcircuit, including a highfrequency choke coil 56, coupled to selectorcircuit 41', 50, 5| through a condenser 51. For developing a controlbias for control tube 53, there is provided an amplifier 58 having inputelectrodes coupled directly to the input circuit of radio-frequencyamplifier 52 through the coupling condenser 59. Coupled to the outputelectrodes of amplifier 58 through a broadband transformer 62, 63 is adiode 64 having a load resistance 65 across which is developed acontrolbias potential variable in accordance with received signalintensities. The voltage developed across the resistor 65 is appliedthrough a filter circuit comprising series resistors 66 and shuntcondenser 61 to the control grid of tube 53 to vary the transconductancethereof in accordance with the amplitude of the input to radio-frequencyamplifier 52. Battery 64' provides an initial large bias potential forthe grid of control tube 53 to make it inoperative in the absence ofsignal-derived bias potentials across resistor 65. The auxiliary diode68, 69 of tube 58 serves to limit the control voltage applied to controltube 53 over filter circuit 66, 61. tential which varies in accordancewith that utilized in conventional A. V. C. circuits is developed acrossa load resistor 10 by means of auxiliary diode 68, H of tube 58 and isapplied negatively through a circuit comprising resistor 12 to thecontrol grid of radio-frequency amplifier 52 automatically to controlits amplification. A cathode-biasing resistor 55, by-passed by condenser55', is provided for tube 53 and a cathode-biasing resistor 68,by-passed by condenser Si, is provided for tube 58.

It is believed that the operation of the circuit of Fig.2 will beapparent from the description given with respect to the circuit ofFig. 1. The normal feed-forward coupling path between the antenna andthe input electrodes of radio-frequency amplifier tube 52 comprisestuned circuits ll, 48, 49, and 41', 50, 5| coupled by the tertiaryportions of coils 41 and 41' and the resistor 52'. The auxiliaryparallel feed-forward coupling of opposite phase comprises vacuum tube53, the transconductance of which is controlled by means of the voltagederived from the input circuit of radio-frequency amplifier 52 by meansof amplifier 58 and rectifier 64. The diode 68, 68 serves to limit thetransconductance of tube 53 of Fig. 2 in the same manner that diode 44,45

of Fig. 1 limits the transconductance of tube 24.

. While there have been described what are at present considered to bethe preferred embodipr" 'lllg an input circuit in the signal-translatingrnents of the invention, it Will be obvious to those skilled in the artthat various changes and modications may be made thereinwithoutdeparting from the invention, and it is, therefore, aimed in theappended claims to cover all such changes and modifications as fallwithin the true spirit and scope of the invention.

What is claimed is:

i. In a modulated-carrier signal receiver comprising an input circuitand a vacuum tube having an input circuit in the signabtranslatingchannel of the receiver and comprising the first signahtranslating tubeof the receiver, an attenuator comprising a first feed-forward couplingcircuit for coupling said input circuit of said receiver to said inputcircuit of said vacuum tube, a second l'ccddorivard coupling circuit.substantially nonlrequencyselective over the tunins; range i thereceiver and including a repeater and terminal circuits individually coupled to said input circuit of said receiver and to said input circuit ofsaid vacuum tube, said second coupling circuit and its terminal.circuits providing a system substantially nonirecuencwselective over thesideband range of received signals, said second coupling circuit and itscoupling to the input circuit of said tube comprising means fordeveloping therein potentials opposite in phase to those developedtherein by said first coupling circuit, and means for controlling therepeating ratio of said second coupiling circuit in accordance With theamplitude of received signals.

2 In a modulated-carrier signal receiver c0m- Jg an input circuit and avacuum tube havchannel of said receiver and comprising the first signaltranslating tube of the receiver, an attcnuator comprising a firstfeed-forward coupling circuit for coupling said input circuit of saidreceiver to said input circuit of said vacuum tube, a secondfeed-forward coupling circuit comprising terminal circuits and avacuumtube signal repeater, said terminal circuits individually beingcoupled to said input circuit of said receiver and said input circuit ofsaid tube, said second coupling circuit being substantially,nonfrequency-selective and being poled to provide a coupling oppositein phase to that of said first coupling circuit, said second couplingcircuit and its terminal circuits providing a system substantiallynonfrequency-selective over the sideband range of received signals, andmeans for controlling the transconductance of said repeater inaccordance with the amplitude of received signals.

3. In a modulated-carrier signal receiver comprising an input circuit, aplurality of vacuum tubes the first one of which has an input circuit,and a plurality of signal selectors in the signaltranslating channel ofsaid receiver, an attenuator comprising a first feed-forward couplingcircuit comprising one of said signal selectors for coupling said inputcircuit of said receiver to i said input circuit of said first one ofsaid vacuum tubes, said one of said selectors being lessirequency-selective than others of said selectors, a second feed-forwardcoupling circuit comprising terminal circuits and comprising avacuum-tube signal repeater, said terminal circuits individually beingcoupled to said input circuit of said receiver and said input circuit ofsaid one of said tubes, said second coupling circuit being poled toprovide a coupling opposite in phase to that of said first couplingcircuit and being substantially nonfrequency-selective, said secondcoupling circuit and its terminal circuits providing a system which issubstantially nonfrequency-selective over the sideband frequency rangeof received signals, and means for controlling the transconduotance ofsaid signal repeater in accordance With the amplitude of the input tosaid one of said vacuum tubes.

4. In a modulated-carrier signal receiver for operation over a givenfrequency range comprising an input circuit and a first vacuum tubehaving an input circuit in. the signal-translating channel of saidreceiver, an attenuator comprising a first feed-forward coupling circuitfor coupling said input circuit of said receiver to said inputcircuit ofsaid vacuum tube, a second feedforward coupling circuit comprisingterminal circuits and comprising a vacuum-tube signal repeater, saidterminal circuits individually being coupled to said input circuit ofsaid receiver and said input circuit of said tube, said second couplingcircuit being substantially nonfrequencyselective and being poled toprovide a coupling opposite in phase to that of said first couplingcircuit, said second coupling circuit and its terminal circuits beingsubstantially nonfrequencyselective over the sideband frequency range ofreceived signals, and means controlling the transconductance of saidsignal repeater in accordance with the amplitude of received signals. 5.In a modulated-carrier signal receiver comprising an antenna, aplurality of signal selectors, and a plurality of vacuum tubes havinginput circuits in the signal-translating channel of said receiver, anattenuator comprising a first leed-forward coupling circuit comprisingone of said signal selectors which is less selective than others of saidsignal selectors for coupling said antenna to the input circuit of thefirst of said vacuum tubes, a second 'feed-forvvard coupling circuitcomprising a vacuum-tube signal repeater and terminal circuitsindividually coupled to said antenna and said input circuit of saidfirst tube, said second coupling circuit being poled to provide acoupling opposite in phase to that of said first coupling circuit, saidsecond coupling a vacuum-tube amplifier coupled to said signaltranslating channel at a point preceding said others of said signalselectors, and means for deriving from the output of said amplifier acontrol-bias potential for said signal repeater variable in accordancewith the amplitude of received.

signals. a

6. In a modulated-carrier signal receiver com-- prising an antenna and aplurality of vacuum tubes having input circuits in thesignal-translating channel of said receiver, an attenuator comprising afirst feed-forward coupling circuit for coupling said antenna to theinput circuit of the first of said vacuum tubes, a second feed forwardcoupling circuit comprising a vacuumtube signal repeater and terminalcircuits individually coupled to said antenna and said input circuit ofsaid first tube, said second coupling circuit being poled to provide acoupling opposite in phase to that of said first coupling circuit, saidsecond coupling circuit and its terminal circuits providing a systemsubstantially nonfrequency-selective over the sideband frequency rangeof received signals, an auxiliary amplifier coupled to the input circuitof said first tube,

rectifying means coupled to said amplifier for deriving a bias potentialvarying in accordance with the amplitude of the desired and undesiredsignal input to said first tube, and means for utilizing said biaspotential to control the transconductance of said vacuum-tube signalrepeater.

7. In a modulated-carrier signal receiver comprising an antenna and aplurality of vacuum tubes having input circuits in thesignal-translating channel of said receiver, an attenuator comprising afirst feed-forward coupling circuit for coupling said antenna to theinput circuit of the first of said vacuum tubes, a second feedforwardcoupling circuit comprising a vacuumtube signal repeater andterminalcircuits individually coupled to said antenna and said inputcircuit of said first tube, said vacuum-tube sig nal repeater beingnormally biased beyond its cutoff point, said second coupling circuitbeing poled to provide a coupling opposite in phase to that of saidfirst coupling circuit, said second coupling circuit and its terminalcircuits providing a system substantially nonfrequency-selective overthe sideband frequency range of received signals, means responsive tothe input to said first tube for developing a bias potential varying inaccordance with the amplitude of the signal input thereto, and means forapplying said potential positively to said vacuum-tube signal repeaterto vary its transconductance.

8. A modulated-carrier signal receiver comprising an antenna, aplurality of vacuum-tube amplifiers having input circuits in] thesignaltranslating channel of the receiver, a first feed forward couplingcircuit for coupling said antenna to the input circuit of the first ofsaid vacuum tubes, a second feed-forward coupling circuit comprising avacuum-tube signal repeater comprising terminal circuits individuallycoupled to said antenna and said input circuit of said first tube, saidsecond coupling circuit being poled to provide a coupling opposite inphase to that of said first coupling circuit, said second couplingcircuit and its terminal circuits providing a system substantiallynonfrequency-selective over the sideband frequency range of receivedsignals, means responsive to the signal input to said first tube fordeveloping a bias potential varying in accordance with the amplitude ofthe input to said first tube, means for applying at least a portion ofsaid potential positively to said vacuum-tube signal repeater to varyits transconductance, and means for applying at least a portion of saidpotential negatively to one or more of said vacuum-tube amplifiers as anautomatic amplification control bias.

9. In a modulated-carrier signal receiver comprising an input circuitand a plurality of vacuum tubes having input circuits in thesignaltranslating channel of said receiver, an attenuator comprisingparallel feed-forward coupling circuits between said input circuit ofsaid receiver and said input circuit of the first of said vacuum tubes,one of said parallel coupling paths comprising a vacuum-tube signalrepeater and terminal circuits, said one of said paths being poled toprovide a coupling opposite in phase and lesser in magnitude than thatof said first coupling circuit, said one of said coupling paths and itsterminal circuits providing a system substantiallynonfrequency-selective over the range of sideband frequencies ofreceived signals, and means for controlling the transconductance of saidrepeater in accordance with the amplitude of received signals.

10. In a modulated-carrier signal receiver comprising an input circuitand a vacuum tube having an input circuit in the signal channel of saidreceiver and comprising the first signaltranslating tube of thereceiver, an attenuator comprising a first feed-forward coupling circuitfor coupling said input circuit of said receiver to said input circuitof said vacuum tube, a second feed-forward coupling circuit comprising avacuum-tube signal repeater and terminal circuits individually coupledto said input circuit of said receiver and said input circuit of saidtube, said second coupling circuit being poled to provide a couplingopposite in phase to that of said first coupling circuit, said secondcoupling circuit and its terminal circuits providing a systemsubstantially nonfrequency-selective over the sideband frequency rangeof received signals, means for controlling the transconductance of saidvacuum-tube repeater in accordance with the amplitude of receivedsignals, and means for limiting the action of said control means so thatsaid transconductance is not increased beyond a predetermined value.

11. In a modulated-carrier signal receiver comprising an input circuitand a vacuum tube having an input circuit in the signal channel of saidreceiver and comprising the first signaltranslating tube of thereceiver, an attenuator comprising a first feed-forward coupling circuitfor coupling said input circuit of said receiver ond feed-forwardcoupling circuit comprising a uacuum-tube signal repeater comprisingterfier means for controlling the transconductance of said signalrepeater in accordance with the amplitude of received signals, and meansoperative to by-pass said rectifier means at a predetermined outputthereof, thereby to limit the action of said control means so that saidtransconductance is not increased beyond a predetermined value.

, DANIEL E. HARNE'IT.

JOHN F. EARRING-TON.

